Blood tissue exchange across mammalian capillaries and venules is a critical cardiovascular function. Yet, the mechanisms underlying transvascular transport under physiologic and pathophysiologic conditions are unclear. The purpose of this research is to determine the permeability characteristics of the microvascular exchange vessels in mammalian skeletal and cardiac muscle. Both fluid and solute permeability coefficients will be determined in vivo by direct measurement of transvascular fluxes in single vessels. Both basic and applied questions will be addressed. In mammalian skeletal muscle: Is there an axial gradient in fluid permeability along capillaries and venules? What are the magnitudes of the capillary and venular exchanging surface areas? What are the single vessel permeability coefficients to small fluorescent solutes? How does solute charge influence transport? What are the effects of ischemia and reduced plasma concentration on single vessel transport in mammals? Studies in the heart are aimed at determining normal transport coefficients as well as examining the effects of varying perfusion pressure and protein concentration on transcoronary exchange. The resulting transport coefficients will be combined with the surface area determinations for comparison with whole organ studies. Also, equivalent pore dimensions will be calculated from the fluid and solute transport coefficients for comparison with both whole organ and ultrastructural studies. These in-vivo, single vessel investigations will bridge the gap between whole organ and ultrastructural studies in mammalian skeletal and cardiac muscle. The results of the basic studies on fluid and solute transport will aid in developing a clearer understanding of how and where blood-tissue exchange occurs. The pathophysiologic studies will resolve the role of colloids and the effects of altered perfusion pressure and total ischemia on transvascular exchange. These studies will also establish a firm base of information for future exchange studies in mammals.